<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .box {
      width: 300px;
      height: 180px;
      background-color: burlywood;
      /* 最大是短边的一半，超过没有效果；如果设置百分比则长宽都是一半，长方形就变成了椭圆 */
      border-radius: 90px;
    }

    .custom {
      width: 300px;
      height: 180px;
      background-color: burlywood;
      /* top-left 左上角开始顺时针自定义 */
      border-radius: 10px 20px 30px 40px;
    }
  </style>
</head>

<body>
  <!-- CSS3 新特性：圆角边框 -->
  <div class="box"></div>
  <br>
  <div class="custom"></div>
</body>

</html>